Promotion of embryonic cortico-cerebral neuronogenesis by miR-124

Glutamatergic neurons of the murine cerebral cortex are generated within periventricular proliferative layers of the embryonic pallium, directly from apical precursors or indirectly, via their basal progenies. Cortical neuronogenesis is the result of different morphogenetic subroutines, including precursors proliferation, death, changes of histogenetic potencies, postmitotic neuronal differentiation. Control of these processes is extremely complex and numerous polypeptide-encoding genes are involved in it. Moreover, a large number of so-called “noncoding genes” are expressed in the developing cortex too. At the moment, their implication in corticogenesis is subject of intensive functional studies. A subset of them encodes for miRNAs, a class of small RNAs with complex biogenesis, regulating gene expression at multiple levels, modulating histogenetic progression and implicated in refinement of positional information. Among cortical miRNAs, there is miR-124. It has been consistently shown to promote neuronogenesis progression, in a variety of experimental contexts. Some aspects of its activity - however - are still controversial, some have to be clarified. An in depth in vivo characterization of its function in the embryonic mammalian cortex is still missing. In this study, by integrating LNA-oligo in situ hybridization, electroporation of stagespecific reporters and immunofluorescence, we reconstructed the cortico-cerebral miR-124 expression pattern, during direct neuronogenesis from apical precursors and indirect neuronogenesis, via basal progenitors. We found that miR-124 expression profile in the developing embryonic cortex includes an abrupt upregulation in apical precursors undergoing direct neuronogenesis as well as a two-steps upregulation in basal progenitors, during indirect neuronogenesis. Differential post-transcriptional processing seems to contribute to this pattern. Moreover, we investigated the role of miR-124 in embryonic corticogenesis by gain-of-function approaches, both in vitro, by lentivirus-based gene transfer, and in vivo, by in utero electroporation. Following overexpression of miR-124, both direct neuronogenesis and progression of neural precursors from the apical to the basal compartment were stimulated. Two are the main conclusions of this study. First, miR-124 expression is progressively upregulated in the mouse embryonic neocortex, during the apical to basal transition of neural precursor cells and upon their exit from cell cycle. Second, miR-124 is involved in fine regulation of these processes.